Self-calibrating, multi-camera machine vision measuring system

ABSTRACT

An apparatus and method for calibrating machine vision measuring systems that have more than one camera are disclosed. A first calibration target is mounted in a fixed relationship to a first camera of the machine vision measuring system. A third camera mounted in a fixed relationship to a second camera of the machine vision measuring system. Second and third calibration targets are mounted in a fixed relationship to one another and viewable by the first camera and by the third camera. A data processor is programmed to compute calibration of the first camera and the second camera, based on a position of the second calibration target relative to the third calibration target and based on a position of the first camera with respect to the third camera. The apparatus and method provide a way to continuously measure the positions of two or more cameras used in the measuring system, and to use such measurements to calibrate the system. If the cameras move with respect to each other, their respective positions are calculated and used in subsequent measurements. The apparatus and method enable a machine vision measuring system to be used without field calibration at the time of installation.

This application is a divisional of application Ser. No. 09/576,442,filed May 22, 2000.

FIELD OF THE INVENTION

The present invention generally relates to calibrating machine visionmeasuring systems that have more than one camera, and relates morespecifically to apparatus and methods that provide automaticself-calibration of computer-aided, three-dimensional aligners for motorvehicle wheels.

BACKGROUND OF THE INVENTION

Machine vision measuring systems that have more than one camera are usedin many applications. For example, wheels of motor vehicles may bealigned on an alignment rack using a computer-aided, three-dimensional(3D) machine vision alignment apparatus and a related alignment method.Examples of methods and apparatus useful in 3D alignment of motorvehicles are described in U.S. Pat. No. 5,724,743, Method and apparatusfor determining the alignment of motor vehicle wheels, and U.S. Pat. No.5,535,522, Method and apparatus for determining the alignment of motorvehicle wheels. The apparatus described in these references is sometimescalled a “3D aligner” or “aligner.”

To determine the alignment of the motor vehicle wheels, such 3D alignersuse cameras that view targets affixed to the wheels. These alignersgenerally require a calibration process to be performed after thealigner is initially installed at the work site. In order to accuratelydetermine the position between the wheels on one side of the vehicle andthe wheels on the other side of the vehicle, the aligner must know whereone camera is positioned with respect to the other camera. According toone calibration method, a large target is positioned in the field ofview of the cameras, typically along the centerline of the alignmentrack, and away from the cameras. Information obtained from each camerais then used to determine the relative positions and orientations of thecameras. Since each camera indicates where the target is with respect toitself, and since each is viewing the same target, the system cancalculate where each camera is located and oriented with respect to theother. This is called a relative camera position (RCP) calibration.

Such calibration allows the results obtained from one side of thevehicle to be compared to the other. Thus, by mounting the two camerasrigidly with respect to each other and then performing an RCPcalibration, the system can be used to locate the wheels on one side ofthe vehicle with respect to the other side of the vehicle from thatpoint on. The RCP transfer function is used to convert one camera'scoordinate system into the other camera's coordinate system so that atarget viewed by one camera can be directly related to a target viewedby the other camera. One approach for performing an RCP is disclosed inU.S. Pat. No. 5,809,658, entitled “Method and Apparatus for CalibratingCameras Used in the Alignment of Motor Vehicle Wheels,” issued toJackson et al. on Sep. 22, 1998.

While RCP calibration is accurate, it requires special fixtures and atrained operator to perform. Thus, there is a need for an easier,simpler calibration process for an aligner.

Further, even after calibration is performed, the aligner may losecalibration over time. The aligner disclosed in the foregoing referenceshas cameras mounted on a boom that is designed to minimize loss ofcalibration. However, if the cameras are jarred or dismounted, or if theboom itself is bent, the aligner will lose calibration. The alignercannot detect loss of calibration itself. Loss of calibration normallyis not detected unless the technician performs a calibration check or afull calibration. A long time may elapse before the technician realizesthat the aligner is out of calibration.

In addition, the boom is large, expensive and presents an obstacle tovehicles entering and leaving the alignment rack. “Drive-through”alignment approaches may be used wherein a vehicle is driven forwardinto a service facility, aligned, and then driven forward to exit theservice facility. This enables other motor vehicles to queue up behindthe vehicle being serviced, improving the speed and efficiency ofalignment services. In one approach of drive-through alignment that hasa rigid boom, it is necessary to raise the camera boom out of the way aseach vehicle passes through. This can be time-consuming, costly, andclumsy.

Based on the foregoing, there is a clear need in this field for anapparatus and method that provides for automatic self-calibration ofmachine vision measuring systems that have more than one camera.

There is also a need for an aligner that may be installed at analignment service facility without calibration at the installation site,thereby eliminating extra hardware and the need for a trained operator.

There is also a need for an aligner that can automatically re-calibrateitself if its cameras are jarred or dismounted, or if the boom is bent.

There is also a need for an aligner that may be re-calibrated quicklywhen a technician determines that the aligner was measuring incorrectly,or when a technician suspects that the relative position of cameras ofthe aligner has changed.

It would also be advantageous to have a 3D aligner that would notrequire a rigid mounting boom for operation, thereby enablingdrive-through alignment without the need to raise the beam and cameras.

SUMMARY OF INVENTION

The foregoing needs and objects, and other needs that will becomeapparent from the following description, are fulfilled by embodiments ofthe present invention, which comprise, in one aspect, an apparatus forcalibrating a machine measuring system. In one embodiment, the machinemeasuring system, having a first camera and a second camera, comprises afirst calibration target mounted in a predetermined relationship to thefirst camera of the machine vision measuring system, and a third cameramounted in a predetermined relationship to the second camera of themachine measuring system. The calibration target is viewed from thethird camera. A data processor is configured to compute a relativecamera position value of the machine measuring system based on arelative position of the first calibration target to the third camera;wherein the relative camera position value represents the relativeposition of the first camera to the second camera. This calibration canbe done frequently, for example, each time that the first and secondcamera measures items of interest, such as wheel targets.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a schematic top plan view of a 3D motor vehicle alignmentsystem.

FIG. 2 is a diagram of upright elements of an alignment system.

FIG. 3A is a diagram of an apparatus that may be used in the step ofmeasuring the relative target position in a method for measuring andcalibrating the relative position of an alignment camera and acalibration camera.

FIG. 3B is a diagram of a view seen by a camera.

FIG. 3C is a diagram of an apparatus that may be used in the step ofmeasuring the relative camera position in a method for measuring andcalibrating the relative position of an alignment camera and acalibration camera.

FIG. 3D is a diagram of views seen by an alignment camera and acalibration camera.

FIG. 4A is a diagram of an apparatus that may be used in a method formeasuring and calibrating the relative position of an alignment cameraand a calibration target.

FIG. 4B is a diagram of a view seen by a setup camera of the apparatusof FIG. 4A.

FIG. 4C is a diagram of a view seen by an alignment camera of theapparatus of FIG. 4A.

FIG. 5A is a diagram of a view of two wheel targets as seen by a firstalignment camera of the apparatus of FIG. 1.

FIG. 5B is a diagram of a view seen by a calibration camera of theapparatus of FIG. 1 during calibration.

FIG. 5C is a diagram of a view of two wheel targets as seen by a secondalignment camera of the apparatus of FIG. 1.

FIG. 6A is a flow diagram illustrating a process of calibrating a cameramodule having two cameras.

FIG. 6B is a flow diagram illustrating a process of calibrating a cameramodule having a camera and a calibration target.

FIG. 6C is a flow diagram of an alignment process that includes carryingout camera calibration during an alignment.

FIG. 7 is a block diagram of a computer system with which an embodimentmay be implemented.

FIG. 8 is a simplified diagram of geometrical relationships of camerasand coordinate systems that provides a basis for computer-basedmathematical computation of numeric values used in the above-describedsystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A method and apparatus for automatic calibration of a machine visionmeasuring system that has more than one camera is described. In thefollowing description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances, well-knownstructures and devices are shown in block diagram form in order to avoidunnecessarily obscuring the present invention.

Structural Overview

FIG. 1 is a schematic top plan view of certain elements of acomputer-aided, 3D motor vehicle wheel alignment system (“aligner”)generally comprising a left camera module 2 and a right camera module 4that are used to align wheels of a motor vehicle. Such an aligner is anexample of a machine vision measuring system that has more than onecamera, however, the present invention is not limited to the context ofa motor vehicle aligner; it is equally applicable to any machine visionmeasuring system that has more than one camera or any machine measuringsystem that has more than one measuring device. In addition, the terms“left” and “right” are used for convenience, and are not intended torequire a particular element to be located in a particular location orrelationship with respect to another element. Any element that is statedto be a “left” element may be placed in a “right” location, and theconverse is also true.

Arrow 30 schematically represents a motor vehicle undergoing alignment.The vehicle includes left and right front wheels 22L, 22R and left andright rear wheels 24L, 24R. An alignment target 80 a, 80 b, 80 c, 80 dis secured to each of the wheels 22L, 22R, 24L, 24R, respectively. Eachalignment target generally comprises a plate 82 on which targetinformation is imprinted and a clamping mechanism 88 for securing thetarget to a wheel.

The left camera module 2 comprises a left alignment camera 10L and acalibration camera 20. Left alignment camera 10L faces the vehicle andviews the left side targets 80 a, 80 b along axis 42. The left alignmentcamera 10L may serve as one of the alignment cameras in the alignerdescribed in U.S. Pat. No. 5,724,743, Method and apparatus fordetermining the alignment of motor vehicle wheels, and U.S. Pat. No.5,535,522, Method and apparatus for determining the alignment of motorvehicle wheels. Camera 10L is rigidly mounted to left rigid mount 12.

A calibration camera 20 faces the right camera module 4 and views acalibration target 16 along axis 46. The calibration camera 20 also isaffixed rigidly to mount 12. In one embodiment, axis 42 and axis 46subtend an angle of about 90 degrees; however, this particular angularrelationship is not required or necessary.

In this exemplary embodiment, calibration camera 20 is illustrated asforming a part of left camera module 2. However, the calibration camera20 also may be configured as part of right camera module 4, in whichcase its view would be directed leftward toward left camera module 2.

Right camera module 4 comprises a right camera 10R that faces thevehicle and functions as a second alignment camera in a 3D alignmentsystem. Right camera 10R is affixed to a rigid camera mount 14.Calibration target 16 is rigidly affixed to camera mount 14 in aposition visible to calibration camera 20 along axis 46.

Calibration camera 20 and left camera 10L are fixed in predetermined,known positions. Similarly, right camera 10R and calibration target 16are fixed in pre-determined, known positions. Thus, the relativeposition of calibration camera to left camera 10L is known, and therelative position of right camera 10R to calibration target 16 is alsoknown. The relative positions of the two cameras contained in the leftcamera module can be obtained by using precision camera mountinghardware. Another approach would be to factory calibrate the two camerapositions and store them for later use.

The mounting of left camera 10L and calibration camera 20 to left mount12 is required to be stable to avoid introduction of calibration errors,which could arise if the cameras move with respect to the mount.Similarly, the mounting of right camera 10R and calibration target 16 tomount 14 is required to be stable.

For illuminating the calibration target 16 and wheel targets 80 a-80 d,left camera module 2 and right camera module 4 further may compriselight sources 62, 64, 66. In one embodiment, a first light source 62 isaligned perpendicular to axis 46 to direct light along that axis toilluminate calibration target 16; a second light source 64 is alignedperpendicular to axis 42 to direct light along that axis to illuminateleft side wheel targets 80 a, 80 b; and a third light source 66 isaligned perpendicular to axis 44 to direct light along that axis toilluminate right side wheel targets 80 c, 80 d. In one embodiment, eachof the light sources 62, 64, 66 comprises a circuit board or othersubstrate on which a plurality of light-emitting diodes (LEDs) aremounted, facing the direction of illumination. However, any other lightsource may be used.

FIG. 2 is a diagram of an alternate embodiment in which an alignmentsystem includes a left upright 52 and a right upright 54. Each upright52, 54 may comprise a rigid post that is affixed to an alignment rack orto the floor of a service facility. Left alignment camera 10L andcalibration camera 20 are mounted within left upright 52, which servesas a protective enclosure and a rigid mount. The cameras may view themotor vehicle under alignment and the calibration target 16 throughsuitable apertures or windows in the upright 52. Right alignment camera10R is mounted and enclosed within right upright 54, and camera 10R mayview the vehicle through a suitable aperture or window in right upright54.

Calibration target 16 may be affixed to an outer surface of upright 54in a position visible to calibration camera 20. Alternatively,calibration target 16 may be affixed within the upright 54 and viewed bycalibration camera 20 through a suitable aperture or window in upright54.

Light sources 62, 64, 66 may be affixed to exterior surfaces of uprights52, 54.

Overview of Calibrating the First Camera Module (the First and ThirdCameras)

Before the aligner can be used, the relative positions of the componentsof each of the camera modules or pods (one pod having the first andthird cameras, a second pod having the second camera and calibrationtarget) must be determined.

If the rigid mount 12 is manufactured to high tolerances (e.g., 0.01″and 0.01°), then the relative positions of the left camera 10L and thecalibration camera 20 are known and there is no need to calibrate therelative positions of the two cameras. Their relative positions will beknown and the same for all assemblies. However, as a method of reducingthe cost, the relative positions of each of the cameras or target in thepods may be calibrated or measured.

FIG. 6A is a flow diagram of a process of calibrating a first cameramodule of a machine vision measurement system having more than twocameras.

In general, in one embodiment calibration of the left camera module 10Linvolves placing two targets that are rigidly mounted to one another inthe field of view of one of the cameras. That camera could be either oneof the three cameras or for the interest of easy set up formanufacturing any other camera. The computer calculates the relativepositions of the two targets (RTP). Then, the targets are moved so thatthe first camera sees one target, and the third camera sees secondtarget. Measurements of the target positions are computed. Based on theRTP and the just measured positions of the targets, the positions of thefirst camera and the third camera are computed.

This sub-process of calibrating the relative position of an alignmentcamera and the calibration camera is now described with reference toFIG. 3A, FIG. 3B, FIG. 3C, FIG. 3D, and FIG. 6A.

In block 602, the left camera module that has the first camera and acalibration camera is set up. In block 604, targets are set up in viewof the left camera. For example, FIG. 3A is a diagram of an apparatusthat may be used in a method for measuring and calibrating the relativeposition of the left alignment camera 10L and the calibration camera 20.A target assembly 70 comprises two targets 72, 74 rigidly secured in aframe 76. The target assembly is placed along axis 42 within the fieldof view of left alignment camera 10L such that both targets 72, 74 arevisible to the camera. In this arrangement, camera 10L will see targets72, 74 in approximately the configuration shown in FIG. 3B. Image 90 isproduced by camera 10L, and includes target images 92 of targets 72, 74.

In block 606, a relative target position value is computed. For example,using known machine vision techniques, a data processor programmedaccording to appropriate software may receive image 90 and measure thelocation of each target 72, 74 based on target images 92. The dataprocessor may then calculate a relative target position (RTP) value foreach of targets 72, 74.

In block 608, the targets are set up such that one target is in view ofthe first camera, and another target is viewed by the calibrationcamera. For example, referring to FIG. 3C, the target assembly is movedsuch that one target 72, 74 is viewed by the left alignment camera 10Land the calibration camera 20, respectively. Movement of the targetframe may be carried out manually by a technician who is performing thecalibration, or using a motorized apparatus. In this position, the leftalignment camera 10L forms an image similar to image 90 shown in FIG.3D. Image 90 includes a target image 94 that represents the view bycamera 10L of target 72. Current target location values are computed, asindicated by block 610. For example, a location value for each target72, 74 with respect to each of the cameras is computed, using machinevision image analysis techniques.

The position of the first camera relative to the calibration camera isthen computed, as shown by block 612. For example, based on the RTPvalue and the target location values, a value representing the relativecamera position of the left alignment camera 10L with respect to thecalibration camera 20 (“RCP Left Module value”) is computed.

Overview of Calibrating Second Camera Module (The 2^(nd) Camera and theCalibration Target)

A process for calibrating a pod or module that contains a camera and atarget, e.g., right camera module 10R, is now described with referenceto the flowchart of FIG. 6B. As shown in block 613, right camera module10R is first set up. Right camera module 10R could be manufactured tohigh tolerances, but to reduce cost, a calibration approach involvingmeasuring relative positions may be used. Generally, as illustrated inFIG. 4A, FIG. 4B, and FIG. 4C, a datum target is placed in view of thesecond camera. An additional camera (“set-up camera”) is placed in aposition to view both the calibration target and the datum target. Theset-up camera in coordination with the computer measures the RTP of thetwo targets. The second camera measures the position of the datumtarget, and the computer determines the position of the calibrationtarget with respect to the second camera, based on the RTP values.

FIG. 4A is a diagram of an apparatus that may be used in a method formeasuring, and thus calibrating, the position of right camera 10Rrelative to the calibration target 16. This apparatus may be used whenthe relative position of the right camera and the calibration target arenot known in advance. In one embodiment, the apparatus of FIG. 4A iscreated as part of a factory calibration process of an aligner.

As shown in block 614, a setup camera and an additional target (“datumtarget”) may be placed in position. Datum target 104 is positioned infront of the right camera module 4 so that it is visible by rightalignment camera 10R. An additional camera, setup camera 100, ispositioned to the side of the right camera module 4 so that it can viewboth the datum target 104 and the calibration target 16.

Relative target position values are computed based on the position ofthe datum target and the calibration target, by using the view of thesetup camera, as shown by block 616. For example, FIG. 4B is a diagramof a view 106 as seen by setup camera 100 in the foregoingconfiguration. View 106 includes a first image 16′ of the calibrationtarget and a second image 104′ of the datum target 104. Using this viewas input to a machine vision processing system, the position of datumtarget 104 and calibration target 16 is measured using the setup camera100. These measurements yield values for the relative target positionsof the datum target 104 and calibration target 16 (“RTP Set Up values”).

In block 617, the relative position of the datum target to the secondcamera is obtained. As shown by block 618, a relative camera target toposition value is computed, based on the relative target position setupvalues and the relative location of the datum target to the secondcamera.

For example, FIG. 4C is a diagram of a view 108 as seen by rightalignment camera 10R in the foregoing configuration. View 108 includes a2nd image 104″ of the datum target 104. Using view 108 as input to amachine vision processing system, the position of datum target 104 withrespect to the right alignment camera 10R is measured. Using valuesrepresenting the relative location of datum target 104 to rightalignment camera 10R, and the RTP Set Up values, a relative cameratarget position value (RCTP) is computed. The RCTP value represents therelationship of the right alignment camera 10R to the right calibrationtarget 16.

At this point, the relative position of the left alignment camera 10Land the calibration camera 20 is now known in the form of the RCP LeftModule value. Further, the relative position of the right alignmentcamera 10R to the calibration target 16 is also known in the form of theRCTP value. Since left alignment camera 10L is rigidly mounted withrespect to calibration camera 20, and right alignment camera 10R isrigidly mounted to with respect to calibration target 16, their relativepositions will not change. In one embodiment, the above steps arenormally performed at the manufacturer's site where the aligner systemis manufactured. The aligner system is thus calibrated at themanufacturer' site, as shown in block 620.

Using the System that has been Calibrated at the Manufacturer' Site.

Alignment may be carried out with a system that has been calibrated atthe manufacturer' site. As indicated in FIG. 1, camera modules 2 and 4have been placed in front of the vehicle to be aligned. The left cameramodule 2 is oriented so that left alignment camera 10L can view the leftside of the vehicle and the calibration camera 20 can view calibrationtarget 16 of the right camera module 4. The right camera module 4 hasbeen positioned so that the right alignment camera 10R can view theright side of the vehicle and so that the calibration target 16 isvisible to calibration camera 20, as in FIG. 1.

FIG. 5A is a diagram of a view 110 as seen by left alignment camera 10Lin this configuration while an alignment operation is occurring. View110 includes images of alignment targets 80 a, 80 b that are on the leftwheels of the vehicle undergoing alignment.

FIG. 5B is a diagram of a view 112 as seen by calibration camera 20 inthis configuration. View 112 includes an image 16″ of calibration target16.

FIG. 5C is a diagram of a view 114 as seen by right alignment camera10R. View 114 includes images of alignment targets 80 c, 80 d that areon the right wheels of the vehicle undergoing alignment.

FIG. 6C is a flow diagram of a process of carrying out cameracalibration during a motor vehicle alignment operation, which, in oneembodiment, is performed at a work site. In block 629, an aligner havinga first camera, a second camera, a calibration camera, and a calibrationtarget is setup as described above. In block 630, a motor vehicle wheelalignment operation or process is begun. Block 630 may involve moving avehicle to an alignment rack, attaching wheel targets to the wheels ofthe vehicle, initializing the aligner, viewing wheel targets with thealigner cameras.

In block 632, the calibration camera measures the position andorientation of the calibration target with respect to the calibrationcamera. For example, when the aligner is installed, and at periodicintervals during use or during an alignment of a motor vehicle,calibration camera 20 may measure the position and orientation ofcalibration target 16 with respect to the calibration camera.

In block 634, an RCP value and an RCTP value are obtained, typicallyfrom memory. In one embodiment, these values are computed as describedabove and are stored in memory. Based on these values (the RCP LeftModule value, the RCTP Right Module value, and the cal target position),values representing the relative positions of the left alignment camera10L and the right alignment camera 10R are calculated, as shown in block636. Such values are termed the relative camera position (RCP) of thealigner. The aligner may then look forward at the vehicle and proceed tomeasure the alignment of the vehicle, as shown by block 638.

The calibration process can be carried out in a “background” mode orbackground processor while the computer is carrying out other regularfunctions of an alignment.

Computation of the RCP value may be carried out at any time before,during, or after a vehicle alignment measurement. For example,computation of the RCP value could be carried out multiple times persecond, once per day, at the start or end of the workday, as necessaryto provide accurate alignments.

Variations

In an alternative embodiment, the foregoing apparatus and processes maybe used without measuring the relative positions of calibration camera20 and left alignment camera 10L of the left camera module 2, or thecamera to target positions of the right camera module 4 in the factory,before the aligner is placed in service in a field environment orservice shop. In this alternative, standard field calibration would becarried out, using the RCP procedure described in the above-noted patentreferences for computing the RCP of the first and second cameras, or anequivalent process. Thereafter, calibration camera 20 measures theposition of calibration target 16. Calibration camera 20 periodicallylooks at calibration target 16 and measures its relative position. Ifsuch measurement indicates a change in the relative position ofcalibration camera 20 and target 16, then the left camera module 2 hasmoved with respect to the right camera module 4. The amount of changemay be used to re-compute and update the RCP value for the aligner.

In still another alternative embodiment, to further simplify theprocess, after the RCP value for the aligner is computed, the relativeposition of the calibration camera 20 and calibration target 16 ismeasured. Periodically, this measurement is compared to the originalmeasurement of the relative position of calibration camera 20 tocalibration target 16, which was done at the time of installation of thealigner. If the two measurements are different, beyond a pre-determinedtolerance, then the aligner informs the operator that the aligner is nolonger calibrated. In response, the operator, or a service technician,may re-perform calibration, for example, using the RCP method.

In still another alternative embodiment, the aligner is provided withmore than two (2) alignment camera modules. For each additionalalignment camera module, the apparatus includes an additionalcalibration camera and calibration target. Each additional alignmentcamera module is calibrated according to the foregoing process, withadditional processing steps to calibrate the additional module. Providedthat each camera in each additional module is rigidly mounted withrespect to its associated calibration target, the entire apparatus maybe calibrated automatically.

In still another embodiment, the calibration camera and calibrationtarget are mounted on different measuring modules. This configurationmay be used with a non-contact aligner that uses one or more lasersystems for determining whether wheels are in alignment.

Further, the processes described herein may be used in embodiments thatuse elements other than a camera to carry out the functions ofcalibration camera 20. Video cameras may be used in an embodiment, butare not required, and any suitable image-capturing device or nayconventional measuring device may be used. For example, gravity gaugesor string gauges may be arranged to detect movement of one or more ofthe alignment cameras 10L, 10R with respect to one another or a fixedpoint. Alternatively, an LED light source may be affixed to one cameramodule to direct a light beam at a detector that is mounted on theopposite camera module. The detector determines a point of maximum lightintensity over the detector surface, and if that point moves over time,then the cameras are determined to have moved, and the RCP value isupdated, or a flag is set to indicate that the system is out ofcalibration.

Computer-Based Mathematical Computations

FIG. 8 is a simplified diagram of geometrical relationships of camerasand coordinate systems that provides a basis for computer-basedmathematical computation of numeric values used in the above-describedsystem.

In FIG. 8, CSA (Coordinate System A) identifies a firstthree-dimensional coordinate system associated with a first alignmentcamera. CSB identifies a second coordinate system associated with asecond alignment camera. CSW identifies a left wheel coordinate systemthat is used for reference purposes. CA is the vector from the origin ofCSW to the origin of CSA. CB is the vector from the origin of CSW to theorigin of CSB. P is a point in space.

PW is the vector from the origin of CSW to P. With respect to CSW, thecomponents of PW are:PWx=PW·xPWy=PW·yPWz=PW·zPW=(PWx*x)+(PWy*y)+(PWz*z)where · indicates a dot product computation.

UA0, UA1, and UA2 are the unit vectors of CSA, i.e., its x, y, and zaxes. With respect to CSW, the components of UA0 are:UA 0 x=UA 0 ·xUA 0 y=UA 0 ·yUA 0 z=UA 0 ·z

The components of UA1, UA2, UB0, UB1, and UB2 may be computed in asimilar manner.

PA is the vector from the origin of CSA to P. With respect to CSA, thecomponents of PA are:PA 0 =PA·UA 0PA 1 =PA·UA 1PA 2 =PA·UA 2PA=(PA 0 *UA 0)+(PA 1 *UA 1)+(PA 2 *UA 2)

PB is the vector from the origin of CSB to P. With respect to CSB, thecomponents of PB are:PB 0 =PB·UA 0PB 1 =PB·UA 1PB 2 =PB·UA 2PB=(PB 0 *UA 0)+(PB 1 *UA 1)+(PB 2 *UA 2)

By vector addition, PW=CA+PA=CB+PBPW=CA+PA$\begin{matrix}{{PWx} = {{{PW} \cdot x} = {{\left( {{CA} + {PA}} \right) \cdot x} = {{{{CA} \cdot x} + {{PA} \cdot x}} =}}}} \\{= {{{\left( {\left( {{PA0}*{UA0}} \right) + \left( {{PA1} + {UA1}} \right) + \left( {{PA2}*{UA2}} \right)} \right) \cdot x} + {CAx}} =}} \\{= {{\left( {{PA0}*\left( {{UA0} \cdot x} \right)} \right) + \left( {{PA1}*\left( {{UA1} \cdot x} \right)} \right) + \left( {{PA2}*\left( {{UA2} \cdot x} \right)} \right) + {CAx}} =}} \\{= {\left( {{PA0}*{UA0x}} \right) + \left( {{PA1} + {UA1x}} \right) + \left( {{PA2}*{UA2x}} \right) + {CAx}}}\end{matrix}\quad$

Therefore,PWx=(PA 0 *UA 0 x)+(PA 1 *UA 1 x)+(PA 2 *UA 2 x)+CAxPWy=(PA 0 *UA 0 y)+(PA 1 *UA 1 y)+(PA 2 *UA 2 x)+CAyPWz=(PA 0 *UA 0 z)+(PA 1 *UA 1 z)+(PA 2 *UA 2 z)+CAx

The foregoing relations may be expressed as a matrix expression of theform: ${\begin{matrix}{PWx} \\{PWy} \\{PWz} \\1\end{matrix}} = {{\begin{matrix}{UA0x} & {UA1x} & {UA2x} & {CAx} \\{UA0y} & {UA1y} & {UA2y} & {CAy} \\{UA0z} & {UA1z} & {UA2z} & {CAz} \\0 & 0 & 0 & 1\end{matrix}}*{\begin{matrix}{PA0} \\{PA1} \\{PA2} \\1\end{matrix}}}$  or, PW=MWA*PA

Accordingly, in one embodiment, using computer storage, a 4×4 matrix ofvalues MWA may be used to completely describe CSA with respect to CSW.The first three column 4-vectors of MWA are the unit 3-vectors of CSAwith respect to CSW, with the fourth component having a value of zero.The last 4-vector of MWA is the 3-vector from the origin (center) of CSWto the origin of CSA, with respect to CSA. Its fourth component has avalue of 1. These 4-vectors and 4×4 matrices are called “homogeneouscoordinates.”

The upper-left 3×3 matrix is just the rotation matrix relating CSA toCSW, and the right-most row is the translation.

Given any point with respect to CSA (i.e., the coordinates of the pointin CSA, which are the components of the vector from the origin of CSA tothe point with respect to the unit vectors of CSA), matrix MWA indicateshow to compute the coordinates of the same point in CSW, namely,multiply PA (the coordinate vector with respect to CSA) by MWA to get PW(the coordinate vector with respect to CSW).

Having rendered values in terms of matrices, matrix mathematics may beused. Specifically, if PW=MWA*PA, then PA=MWA⁻¹*PW.

By the above definitions, the 4×4 matrix MWA⁻¹ completely characterizesor describes CSW with respect to CSA. The foregoing also applies if PAis replaced by PW, and MWA is replaced by MWA⁻¹. To get MWA⁻¹, thefollowing process is used.

1. Transpose the upper left 3×3 matrix.

2. Replace the right-most column vector (CAx, CAy, CAz, 1), the vectorfrom the origin of CSW to CSA, with respect to CSW, with (−CA0, −CA1,−CA2, 1), the vector from the origin of CSA to the origin of CSW—thelatter is in the opposite direction from vector CA, from the origin ofCSW to the origin of CSA—, with respect to CSA:CA 0 =CA·UA 0=(CAx*UA 0 x)+(CAy*UA 0 y)+(CAz*UA 0 z)CA 1 =CA·UA 1=(CAx*UA 1 x)+(CAy*UA 1 y)+(CAz*UA 1 z)CA 2 =CA·UA 2=(CAx*UA 2 x)+(CAy*UA 2 y)+(CAz*UA 2 z)

and ${\begin{matrix}{CA0} \\{CA1} \\{CA2}\end{matrix}} = {{\begin{matrix}{UA0x} & {UA0x} & {UA0z} \\{UA1x} & {UA1y} & {UA1z} \\{UA2x} & {UA2y} & {UA2z}\end{matrix}}*{\begin{matrix}{CAx} \\{CA1} \\{CA2}\end{matrix}}}$

The 3×3 matrix above is the transpose of the upper left 3×3 matrix inthe 4×4 matrix MWA, the one that goes into the upper left 3×3 matrixpositions of MWA⁻¹. Thus: ${MWA}^{- 1} = {\begin{matrix}{UA0x} & {UA0x} & {UA0z} & - & {CA0} \\{UA1x} & {UA1y} & {UA1z} & - & {CA1} \\{UA2x} & {UA2y} & {UA2z} & - & {CA2} \\0 & 0 & 0 & \quad & 1\end{matrix}}$

For purposes of consistent notation, if 4×4 matrix MWA completelycharacterizes or describes CSA with respect to CSW, and MWA⁻¹ does thesame for CSW with respect to CSA, then MWA⁻¹=MAW.

Further, if PW=MWA*PA, and PA=MWA⁻¹*PW=MAW*PW, thenPW=MWB*PB, and PB=MWB ⁻¹ *PW=MBW*PW

Since the same PW is used in both expressions, thenMWA*PA=MWB*PB=PWMWA ⁻¹ *MWA*PA=MWA ⁻¹ *MWB*PB

But since MWA⁻¹*MWA is the identity matrix, thenPA=MWA ⁻¹ *MWB*PB=MAW*MWB*PB=MAB*PB

and therefore,MAB=MWA ⁻¹ *MWB=MAW*MWB

The 4×4 matrix MAB completely characterizes or describes CSB withrespect to CSA. Thus, MAB is the RCP or RTP matrix.

In one exemplary software implementation, a VECTOR structure is definedas an array of three numbers; a MATRIX structure is defined as an arrayof three VECTORs; and a PLANE structure is a MATRIX and a VECTOR. TheMATRIX is the 3×3 rotation matrix, whose VECTORs are the three unitvectors of the coordinate system, and the VECTOR is the vector to theorigin of the coordinate system. All such VECTORs' components areexpressed with respect to a base coordinate system.

In one exemplary function, Plane 1 defined relative to WCS is MWA; Plane2 defined relative to WCS is MWB; Plane 2 defined relative to plane 1 isMAB; Plane 1 defined relative to plane 2 is MBA. Then, a function may bedefined having the API,

void mat3Plane21To2W (PLANE *p1, PLANE *p21, PLANE *p2w) /* Given plane1 defined relative to WCS (MWA) Given plane 2 defined relative to plane1 (MAB) Return plane 2 defined relative to WCS (MWB) */

Using matrix notation: MWB=MWA*MAB ${\begin{matrix}{UA0x} & {UA1x} & {UA2x} & {CAx} \\{UA0y} & {UA1y} & {UA2y} & {CAy} \\{UA0z} & {UA1z} & {UA2z} & {CAz} \\0 & 0 & 0 & 1\end{matrix}}*{\begin{matrix}{UAB00} & {UAB10} & {UAB20} & {CAB0} \\{UAB01} & {UAB11} & {UAB21} & {CAB1} \\{UAB02} & {UAB12} & {UAB22} & {CAB2} \\0 & 0 & 0 & 1\end{matrix}}$

The upper left 3×3 of the product 4×4 matrix MWB is the product of theupper left 3×3 matrix values of MWA and MAB, as a result of the zerovalues in the bottom row of all the 4×4 matrices. The rightmost columnof the product 4×4 matrix MWB is the sum of the product of the upperleft 3×3 of MWA and the rightmost column vector of MAB, and therightmost column vector of MWA, also because of the zero values and theone value in the bottom row of all the 4×4 matrices.

To save computational time, in one embodiment, no multiply operationsare carried out by zero or one for the bottom rows of the 4×4 matrices.Accordingly, 3×3 matrix and 3-vector multiply, add, and transposeoperations are carried out.

Similar functions may be defined for other transformations, as follows:

void mat3Plane2WTo21 (PLANE *p1, PLANE *p2w, PLANE *p21) /* Given plane1 defined relative to WCS (MWA) Given plane 2 defined relative to WCS(MWB) Return plane 2 defined relative to plane1 (MAB) */

Using matrix notation: MAB=MAW*MWB=MWA⁻¹*MWB

void mat3Plane12To2 (PLANE *p1, PLANE *p12, PLANE *p2) /* Given plane 1defined relative to WCS (MWA) Given plane 1 defined relative to plane 2(MBA) Return plane 2 defined relative to WCS (MWB) */

Using matrix notation: MWB=MWA*MAB=MWA*MBA⁻¹

The discussion above observes that a 4×4 matrix of values MWA may beused to completely describe CSA with respect to CSW. Further, theinverse of MWA, matrix MWA⁻¹, completely describes CSW with respect toCSA. Accordingly,MWA⁻¹=MAWMAB=MWA ⁻¹ *MWB=MAW*MWB

Using machine vision analysis techniques, the above-described systemmake take a camera image of a target, e.g., image 90 of FIG. 3B, andcompute the coordinate system of the target with respect to the camera.

A computation of relative target position (RTP) is now described withreference to FIG. 3A. For purposes of computing RTP:

Let CSL be the coordinate system of left camera 10L.

Let CSA be the coordinate system of target 72.

MLA represents CSA with respect to CSL.

Let CSB be the coordinate system of target 74.

MLB represents CSB with respect to CSL.

Given left camera image 90 containing images 92 of targets 72 and 74,machine vision analysis techniques result in creating and storingmatrices MLA and MLB. According, the RTP (between target 72 and 74 )value is given byRTP=MAB=MAL*MLB=MLA ⁻¹ *MLB

Based on this, the system may compute MLA from MLB and the opposite, byMLA=MLB*MBA=MLB*RTP ⁻¹ MLB=MLA*MAB=MLA*RTP

When the value of RTP is created and stored, target assembly 70 may bemoved so that the left camera 10L sees target 72 and the calibrationcamera 20 sees target 74. Let CSC be the coordinate system of thecalibration camera 20. Given left camera image 90, containing image 94of target 70 (FIG. 3D), and calibration camera image 96 containing image98 of target 74 (FIG. 3D), machine vision analysis techniques result increating and storing matrices MLA, which describes CSA with respect toCSL, and MCB, which describes CSB with respect to CSC.

Based on such matrices, the value of the relative camera position RCP ofthe left camera 10L with respect to the calibration camera 20 may becomputed as:MCL=MCB*MBL=MCB*MLB ⁻¹ =MCB*(MLA*RTP)⁻¹ 32 MCB*RTP ⁻¹ *MLA ⁻¹

and the value of the relative camera position RCP of the calibrationcamera 20 with respect to the left camera 10L may be computed as:MLC=MLB*MBC=MLB*MCB ⁻¹ =MLA*RTP*MCB ⁻¹

Now a computation of values for the right camera 10R is presented.Referring now to FIG. 4A,

Let CSS be the coordinate system of the setup camera 100.

Let CSR be the coordinate system of the right camera 10R.

Let CSQ be the coordinate system of the calibration target 16.

Let CSD be the coordinate system of the datum target 104.

Given setup camera image 106 of FIG. 4B, containing images 16′ and 104′of calibration target 16 and datum target 104, machine vision analysistechniques result in creating and storing matrices MSQ, which describesCSQ with respect to CSS, and MSD, which describes CSD with respect toCSS. Further, given right camera image 108, containing image 104″ of thedatum target 104, machine vision analysis techniques result in creatingand storing a matrix MRD, which describes CSD with respect to CSR. MAQdescribes the RTP between the calibration target 16 and the datum target104 (CSD with respect to CSQ).

Then the coordinate system of the calibration target 16 with respect tothe right camera, that is, CSQ with respect to CSR, is given byMRQ=MRD*MAQ=MRD*(MDS*MSQ)=MRD*MSD ⁻¹ *MSQ

Accordingly, a value of MLC, which describes the calibration camera withrespect to the left camera, and MRQ, which describes the calibrationtarget with respect to the right camera, may be computed.

In ordinary operation, the system produces images of the type shown inFIG. 5A, FIG. 5B, FIG. 5C. The left camera 10L produces image 110 (FIG.5A) of the two left wheel targets. Machine vision analysis techniquesresult in creating and storing values for the coordinate systems of thewheel targets in the left camera coordinate system. If the left cameracoordinate system is defined as the world coordinate system, then theleft wheel targets are transposed into the world coordinate system.

Calibration camera 20 produces image 112 (FIG. 5B) of the calibrationtarget. Machine vision analysis techniques result in creating andstoring values for the coordinate system of the calibration target inthe calibration camera coordinate system, MCQ.

The right camera 10L generates image 114 of FIG. 5C of the two rightwheel targets. Machine vision analysis techniques result in creating andstoring values for the coordinate systems of these wheel targets in theright camera coordinate system, MRW wherein “W” refers to “wheel” ratherthan “world”. Generally, the left camera coordinate system serves as theworld coordinate system.

The right wheel target values may be transposed into the same worldcoordinate system as the left wheel targets by computing MLW, the rightwheel targets in the left (world) coordinate system. From thecalibration process, values of MLC and MRQ are known. The systemmeasures MCQ based on the image of FIG. 5B and MRW based on the image ofFIG. 5C. Accordingly,MLW=MLR*MRW=MLC*MCR*MRW=MLC*MCQ*MRQ ⁻¹ *MRW

Hardware Overview

FIG. 7 is a block diagram that illustrates a computer system 700 uponwhich an embodiment of the invention may be implemented. Computer system700 may be used as the arrangement for some or all of device 100 or forthe arrangement of an external computer or workstation that communicateswith device 100.

Computer system 700 includes a bus 702 or other communication mechanismfor communicating information, and a processor 704 coupled with bus 702for processing information. Computer system 700 also includes a mainmemory 706, such as a random access memory (RAM) or other dynamicstorage device, coupled to bus 702 for storing information andinstructions to be executed by processor 704. Main memory 706 also maybe used for storing temporary variables or other intermediateinformation during execution of instructions to be executed by processor704. Computer system 700 further includes a read only memory (ROM) 708or other static storage device coupled to bus 702 for storing staticinformation and instructions for processor 704. A storage device 710,such as a magnetic disk or optical disk, is provided and coupled to bus702 for storing information and instructions.

Computer system 700 may be coupled via bus 702 to a display 712, such asa cathode ray tube (CRT), for displaying information to a computer user.An input device 714, including alphanumeric and other keys, is coupledto bus 702 for communicating information and command selections toprocessor 704. Another type of user input device is cursor control 716,such as a mouse, a trackball, or cursor direction keys for communicatingdirection information and command selections to processor 704 and forcontrolling cursor movement on display 712. This input device typicallyhas two degrees of freedom in two axes, a first axis (e.g., x) and asecond axis (e.g., y), that allows the device to specify positions in aplane.

Embodiments of the invention are related to the use of computer system700 for automatic calibration of an aligner. According to one embodimentof the invention, automatic calibration of an aligner is provided bycomputer system 700 in response to processor 704 executing one or moresequences of one or more instructions contained in main memory 706. Suchinstructions may be read into main memory 706 from anothercomputer-readable medium, such as storage device 710. Execution of thesequences of instructions contained in main memory 706 causes processor704 to perform the process steps described herein. In alternativeembodiments, hard-wired circuitry may be used in place of or incombination with software instructions to implement the invention. Thus,embodiments of the invention are not limited to any specific combinationof hardware circuitry and software.

The term “computer-readable medium” as used herein refers to any mediumthat participates in providing instructions to processor 704 forexecution. Such a medium may take many forms, including but not limitedto, non-volatile media, volatile media, and transmission media.Non-volatile media includes, for example, optical or magnetic disks,such as storage device 710. Volatile media includes dynamic memory, suchas main memory 706. Transmission media includes coaxial cables, copperwire and fiber optics, including the wires that comprise bus 702.Transmission media can also take the form of acoustic or light waves,such as those generated during radio-wave and infra-red datacommunications.

Common forms of computer-readable media include, for example, a floppydisk, a flexible disk, hard disk, magnetic tape, or any other magneticmedium, a CD-ROM, any other optical medium, punch cards, paper tape, anyother physical medium with patterns of holes, a RAM, a PROM, and EPROM,a FLASH-EPROM, any other memory chip or cartridge, a carrier wave asdescribed hereinafter, or any other medium from which a computer canread.

Various forms of computer readable media may be involved in carrying oneor more sequences of one or more instructions to processor 704 forexecution. For example, the instructions may initially be carried on amagnetic disk of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to computer system 700 canreceive the data on the telephone line and use an infra-red transmitterto convert the data to an infra-red signal. An infra-red detector canreceive the data carried in the infra-red signal and appropriatecircuitry can place the data on bus 702. Bus 702 carries the data tomain memory 706, from which processor 704 retrieves and executes theinstructions. The instructions received by main memory 706 mayoptionally be stored on storage device 710 either before or afterexecution by processor 704.

Computer system 700 also includes a communication interface 718 coupledto bus 702. Communication interface 718 provides a two-way datacommunication coupling to a network link 720 that is connected to alocal network 722. For example, communication interface 718 may be anintegrated services digital network (ISDN) card or a modem to provide adata communication connection to a corresponding type of telephone line.As another example, communication interface 718 may be a local areanetwork (LAN) card to provide a data communication connection to acompatible LAN. Wireless links may also be implemented. In any suchimplementation, communication interface 718 sends and receiveselectrical, electromagnetic or optical signals that carry digital datastreams representing various types of information.

Network link 720 typically provides data communication through one ormore networks to other data devices. For example, network link 720 mayprovide a connection through local network 722 to a host computer 724 orto data equipment operated by an Internet Service Provider (ISP) 726.ISP 726 in turn provides data communication services through the worldwide packet data communication network now commonly referred to as the“Internet” 728. Local network 722 and Internet 728 both use electrical,electromagnetic or optical signals that carry digital data streams. Thesignals through the various networks and the signals on network link 720and through communication interface 718, which carry the digital data toand from computer system 700, are exemplary forms of carrier wavestransporting the information.

Computer system 700 can send messages and receive data, includingprogram code, through the network(s), network link 720 and communicationinterface 718. In the Internet example, a server 730 might transmit arequested code for an application program through Internet 728, ISP 726,local network 722 and communication interface 718. In accordance withembodiments of the invention, one such downloaded application providesfor automatic calibration of an aligner as described herein.

The received code may be executed by processor 704 as it is received,and/or stored in storage device 710, or other non-volatile storage forlater execution. In this manner, computer system 700 may obtainapplication code in the form of a carrier wave.

Advantages and Further Variations

The embodiments disclosed in this document are adaptable to othercontexts. In particular, the embodiments are useful in calibrating anymachine vision measuring system that has more than one camera. Further,the embodiments may be used in connection with alignment of arecreational vehicle (RV). An aligner for RVs normally requires a widerboom than a standard aligner. Using the embodiments disclosed in thisdocument, an aligner for RVs may be constructed simply by bolting itsuprights slightly further apart, with no new hardware.

Because the apparatus described above is self-calibrating, it can beincorporated in a portable aligner, because calibration after setup isnot required. A portable alignment operation could be carried out usingtwo cameras on a tripod in a parking lot, garage, or similar environmentwithout the time consuming calibration. Thus, the apparatus could beused to facilitate an entirely new service, remote or on-site alignment.

Further, techniques described above to measure (or calibrate) therelative position of the left camera to the right camera may be employedin a system that has a plurality of devices. These techniques are usedto measure the relative position of one device of the plurality ofdevices with respect to another device of the plurality of devices. Inthese conditions, any pair of devices of the plurality of devices thatincludes a first device and a second device may be treated as the pairof the left camera and the right camera in the above-describedtechniques. A calibration device is then mounted near the first devicewherein the relative position of the calibration device to the firstdevice is predetermined. Similarly, a calibration target is mounted nearthe second device wherein the relative position of the calibrationtarget to the second device is also predetermined. The relative positionof the calibration device to the calibration target is then measured.Finally, the relative position of the first device to the second deviceis calculated based on 1) the relative position of the calibrationdevice to the first device, 2) the relative position of the calibrationtarget to the second device, and 3) the relative position of thecalibration device to the calibration target. In one embodiment, thecalibration device is configured to measure the relative position of thecalibration device to the calibration target.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

1. An apparatus for calibrating a machine measuring system that has afirst measuring device and a second measuring device, wherein the firstmeasuring device and the second measuring device are image-capturingdevices remote to a vehicle to undergo a measurement for capturingimages of at least one target attached to the vehicle, the apparatuscomprising: a first calibration device mounted in a known relationshipto the first measuring device; second calibration device mounted in aknown relationship to the second measuring device, wherein the firstcalibration device and the second calibration device coordinatelygenerate measurements of a relative position between the firstcalibration device and the second calibration device; a memory devicefor storing a reference value that represents a reference position ofthe first calibration device relative to the second calibration device;and a data processor, coupled to the first measuring device, the secondmeasuring device, and at least one of the first calibration device andthe second calibration device, and configured to repeatedly receivesignals representing the relative position between the first calibrationdevice and the second calibration device, and determine vehicleparameters based on the images of the at least one target, the knownrelationship of the first measuring device relative to the firstcalibration device, the known relationship of the second measuringdevice relative to the second calibration device, and based on therelative position between the first calibration device and the secondcalibration device; wherein the data processor is further configured toperiodically compare the reference value with a measurement coordinatelygenerated by the first calibration device and the second calibrationdevice representing an updated relative position between the firstcalibration device and the second calibration device.
 2. An apparatus asrecited in claim 1, wherein the first calibration device comprises alight source mounted in fixed relationship to the first measuring deviceand the second calibration device comprises a light detector mounted infixed relationship to the second measuring device and having an outputcoupled to the data processor.
 3. A computer-readable medium bearinginstructions for calibrating a machine measuring system that has a firstmeasuring device, a second measuring device, first calibration deviceattached to the first measuring device in a known positionalrelationship, second calibration device attached to the second measuringdevice in a known positional relationship and a data processing systemcoupled to the first measuring device, the second measuring device, andat least one of the first calibration device and the second calibrationdevice, wherein the first measuring device and the second measuringdevice are image-capturing devices remote to a vehicle for capturingimages of at least one target attached to the vehicle, and the firstcalibration device and the second calibration device coordinatelygenerate measurements of a relative position between the firstcalibration device and the second calibration device, thecomputer-readable medium comprising instructions, upon execution by thedata processing system, that control the data processing system toperform the steps of: receiving a periodically updated signalrepresenting an updated relative position between the first calibrationdevice and the second calibration device from at least one of the firstcalibration device and the second calibration device; accessing areference value that represents a reference position of the firstcalibration device relative to the second calibration device;determining vehicle parameters based on the images of the at least onetarget, the known positional relationship of the first measuring devicerelative to the first calibration device, the known positionalrelationship of the second measuring device relative to the secondcalibration device, and the relative position between the firstcalibration device and the second calibration device; and periodicallycomparing the reference value with the undated relative position betweenthe first calibration device and the second calibration device. 4.vehicle alignment system comprising: a first image-capturing deviceremote to a vehicle for capturing images of a first target positioned ata first known relationship to the vehicle; a second image capturingdevice remote to the vehicle for capturing images of a second targetpositioned at a second known relationship to the vehicle; firstcalibration device having a known positional relationship relative tothe first image-capturing device; second calibration device having aknown positional relationship relative to the second image-capturingdevice, wherein the first calibration device and the second calibrationdevice coordinately generate measurements of a relative position betweenthe first calibration device and the second calibration device; a dataprocessing system configured to form signal communication with the firstimage-capturing device, the second image-capturing device and at leastone of the first calibration device and the second calibration device,wherein the data processing system includes a data storage devicestoring instructions, upon executed by the data processing system,controlling the data processing system to perform the steps of:receiving a signal representing an updated relative position between thefirst calibration device and the second calibration device from at leastone of the first calibration device and the second calibration device;accessing a reference value that represents a reference position of thefirst calibration device relative to the second calibration device;determining alignment parameters of the vehicle based on the images ofthe first target and the second target, the known positionalrelationship of the first image-capturing device relative to the firstcalibration device, the known positional relationship of the secondimage-capturing device relative to the second calibration device, andthe relative position between the first calibration device and thesecond calibration device; and periodically comparing the referencevalue with the updated relative position between the first calibrationdevice find the second calibration device.